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Source file src/context/context.go

Documentation: context

     1  // Copyright 2014 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  // Package context defines the Context type, which carries deadlines,
     6  // cancelation signals, and other request-scoped values across API boundaries
     7  // and between processes.
     8  //
     9  // Incoming requests to a server should create a Context, and outgoing
    10  // calls to servers should accept a Context. The chain of function
    11  // calls between them must propagate the Context, optionally replacing
    12  // it with a derived Context created using WithCancel, WithDeadline,
    13  // WithTimeout, or WithValue. When a Context is canceled, all
    14  // Contexts derived from it are also canceled.
    15  //
    16  // The WithCancel, WithDeadline, and WithTimeout functions take a
    17  // Context (the parent) and return a derived Context (the child) and a
    18  // CancelFunc. Calling the CancelFunc cancels the child and its
    19  // children, removes the parent's reference to the child, and stops
    20  // any associated timers. Failing to call the CancelFunc leaks the
    21  // child and its children until the parent is canceled or the timer
    22  // fires. The go vet tool checks that CancelFuncs are used on all
    23  // control-flow paths.
    24  //
    25  // Programs that use Contexts should follow these rules to keep interfaces
    26  // consistent across packages and enable static analysis tools to check context
    27  // propagation:
    28  //
    29  // Do not store Contexts inside a struct type; instead, pass a Context
    30  // explicitly to each function that needs it. The Context should be the first
    31  // parameter, typically named ctx:
    32  //
    33  // 	func DoSomething(ctx context.Context, arg Arg) error {
    34  // 		// ... use ctx ...
    35  // 	}
    36  //
    37  // Do not pass a nil Context, even if a function permits it. Pass context.TODO
    38  // if you are unsure about which Context to use.
    39  //
    40  // Use context Values only for request-scoped data that transits processes and
    41  // APIs, not for passing optional parameters to functions.
    42  //
    43  // The same Context may be passed to functions running in different goroutines;
    44  // Contexts are safe for simultaneous use by multiple goroutines.
    45  //
    46  // See https://blog.golang.org/context for example code for a server that uses
    47  // Contexts.
    48  package context
    49  
    50  import (
    51  	"errors"
    52  	"fmt"
    53  	"reflect"
    54  	"sync"
    55  	"time"
    56  )
    57  
    58  // A Context carries a deadline, a cancelation signal, and other values across
    59  // API boundaries.
    60  //
    61  // Context's methods may be called by multiple goroutines simultaneously.
    62  type Context interface {
    63  	// Deadline returns the time when work done on behalf of this context
    64  	// should be canceled. Deadline returns ok==false when no deadline is
    65  	// set. Successive calls to Deadline return the same results.
    66  	Deadline() (deadline time.Time, ok bool)
    67  
    68  	// Done returns a channel that's closed when work done on behalf of this
    69  	// context should be canceled. Done may return nil if this context can
    70  	// never be canceled. Successive calls to Done return the same value.
    71  	//
    72  	// WithCancel arranges for Done to be closed when cancel is called;
    73  	// WithDeadline arranges for Done to be closed when the deadline
    74  	// expires; WithTimeout arranges for Done to be closed when the timeout
    75  	// elapses.
    76  	//
    77  	// Done is provided for use in select statements:
    78  	//
    79  	//  // Stream generates values with DoSomething and sends them to out
    80  	//  // until DoSomething returns an error or ctx.Done is closed.
    81  	//  func Stream(ctx context.Context, out chan<- Value) error {
    82  	//  	for {
    83  	//  		v, err := DoSomething(ctx)
    84  	//  		if err != nil {
    85  	//  			return err
    86  	//  		}
    87  	//  		select {
    88  	//  		case <-ctx.Done():
    89  	//  			return ctx.Err()
    90  	//  		case out <- v:
    91  	//  		}
    92  	//  	}
    93  	//  }
    94  	//
    95  	// See https://blog.golang.org/pipelines for more examples of how to use
    96  	// a Done channel for cancelation.
    97  	Done() <-chan struct{}
    98  
    99  	// If Done is not yet closed, Err returns nil.
   100  	// If Done is closed, Err returns a non-nil error explaining why:
   101  	// Canceled if the context was canceled
   102  	// or DeadlineExceeded if the context's deadline passed.
   103  	// After Err returns a non-nil error, successive calls to Err return the same error.
   104  	Err() error
   105  
   106  	// Value returns the value associated with this context for key, or nil
   107  	// if no value is associated with key. Successive calls to Value with
   108  	// the same key returns the same result.
   109  	//
   110  	// Use context values only for request-scoped data that transits
   111  	// processes and API boundaries, not for passing optional parameters to
   112  	// functions.
   113  	//
   114  	// A key identifies a specific value in a Context. Functions that wish
   115  	// to store values in Context typically allocate a key in a global
   116  	// variable then use that key as the argument to context.WithValue and
   117  	// Context.Value. A key can be any type that supports equality;
   118  	// packages should define keys as an unexported type to avoid
   119  	// collisions.
   120  	//
   121  	// Packages that define a Context key should provide type-safe accessors
   122  	// for the values stored using that key:
   123  	//
   124  	// 	// Package user defines a User type that's stored in Contexts.
   125  	// 	package user
   126  	//
   127  	// 	import "context"
   128  	//
   129  	// 	// User is the type of value stored in the Contexts.
   130  	// 	type User struct {...}
   131  	//
   132  	// 	// key is an unexported type for keys defined in this package.
   133  	// 	// This prevents collisions with keys defined in other packages.
   134  	// 	type key int
   135  	//
   136  	// 	// userKey is the key for user.User values in Contexts. It is
   137  	// 	// unexported; clients use user.NewContext and user.FromContext
   138  	// 	// instead of using this key directly.
   139  	// 	var userKey key
   140  	//
   141  	// 	// NewContext returns a new Context that carries value u.
   142  	// 	func NewContext(ctx context.Context, u *User) context.Context {
   143  	// 		return context.WithValue(ctx, userKey, u)
   144  	// 	}
   145  	//
   146  	// 	// FromContext returns the User value stored in ctx, if any.
   147  	// 	func FromContext(ctx context.Context) (*User, bool) {
   148  	// 		u, ok := ctx.Value(userKey).(*User)
   149  	// 		return u, ok
   150  	// 	}
   151  	Value(key interface{}) interface{}
   152  }
   153  
   154  // Canceled is the error returned by Context.Err when the context is canceled.
   155  var Canceled = errors.New("context canceled")
   156  
   157  // DeadlineExceeded is the error returned by Context.Err when the context's
   158  // deadline passes.
   159  var DeadlineExceeded error = deadlineExceededError{}
   160  
   161  type deadlineExceededError struct{}
   162  
   163  func (deadlineExceededError) Error() string   { return "context deadline exceeded" }
   164  func (deadlineExceededError) Timeout() bool   { return true }
   165  func (deadlineExceededError) Temporary() bool { return true }
   166  
   167  // An emptyCtx is never canceled, has no values, and has no deadline. It is not
   168  // struct{}, since vars of this type must have distinct addresses.
   169  type emptyCtx int
   170  
   171  func (*emptyCtx) Deadline() (deadline time.Time, ok bool) {
   172  	return
   173  }
   174  
   175  func (*emptyCtx) Done() <-chan struct{} {
   176  	return nil
   177  }
   178  
   179  func (*emptyCtx) Err() error {
   180  	return nil
   181  }
   182  
   183  func (*emptyCtx) Value(key interface{}) interface{} {
   184  	return nil
   185  }
   186  
   187  func (e *emptyCtx) String() string {
   188  	switch e {
   189  	case background:
   190  		return "context.Background"
   191  	case todo:
   192  		return "context.TODO"
   193  	}
   194  	return "unknown empty Context"
   195  }
   196  
   197  var (
   198  	background = new(emptyCtx)
   199  	todo       = new(emptyCtx)
   200  )
   201  
   202  // Background returns a non-nil, empty Context. It is never canceled, has no
   203  // values, and has no deadline. It is typically used by the main function,
   204  // initialization, and tests, and as the top-level Context for incoming
   205  // requests.
   206  func Background() Context {
   207  	return background
   208  }
   209  
   210  // TODO returns a non-nil, empty Context. Code should use context.TODO when
   211  // it's unclear which Context to use or it is not yet available (because the
   212  // surrounding function has not yet been extended to accept a Context
   213  // parameter). TODO is recognized by static analysis tools that determine
   214  // whether Contexts are propagated correctly in a program.
   215  func TODO() Context {
   216  	return todo
   217  }
   218  
   219  // A CancelFunc tells an operation to abandon its work.
   220  // A CancelFunc does not wait for the work to stop.
   221  // After the first call, subsequent calls to a CancelFunc do nothing.
   222  type CancelFunc func()
   223  
   224  // WithCancel returns a copy of parent with a new Done channel. The returned
   225  // context's Done channel is closed when the returned cancel function is called
   226  // or when the parent context's Done channel is closed, whichever happens first.
   227  //
   228  // Canceling this context releases resources associated with it, so code should
   229  // call cancel as soon as the operations running in this Context complete.
   230  func WithCancel(parent Context) (ctx Context, cancel CancelFunc) {
   231  	c := newCancelCtx(parent)
   232  	propagateCancel(parent, &c)
   233  	return &c, func() { c.cancel(true, Canceled) }
   234  }
   235  
   236  // newCancelCtx returns an initialized cancelCtx.
   237  func newCancelCtx(parent Context) cancelCtx {
   238  	return cancelCtx{Context: parent}
   239  }
   240  
   241  // propagateCancel arranges for child to be canceled when parent is.
   242  func propagateCancel(parent Context, child canceler) {
   243  	if parent.Done() == nil {
   244  		return // parent is never canceled
   245  	}
   246  	if p, ok := parentCancelCtx(parent); ok {
   247  		p.mu.Lock()
   248  		if p.err != nil {
   249  			// parent has already been canceled
   250  			child.cancel(false, p.err)
   251  		} else {
   252  			if p.children == nil {
   253  				p.children = make(map[canceler]struct{})
   254  			}
   255  			p.children[child] = struct{}{}
   256  		}
   257  		p.mu.Unlock()
   258  	} else {
   259  		go func() {
   260  			select {
   261  			case <-parent.Done():
   262  				child.cancel(false, parent.Err())
   263  			case <-child.Done():
   264  			}
   265  		}()
   266  	}
   267  }
   268  
   269  // parentCancelCtx follows a chain of parent references until it finds a
   270  // *cancelCtx. This function understands how each of the concrete types in this
   271  // package represents its parent.
   272  func parentCancelCtx(parent Context) (*cancelCtx, bool) {
   273  	for {
   274  		switch c := parent.(type) {
   275  		case *cancelCtx:
   276  			return c, true
   277  		case *timerCtx:
   278  			return &c.cancelCtx, true
   279  		case *valueCtx:
   280  			parent = c.Context
   281  		default:
   282  			return nil, false
   283  		}
   284  	}
   285  }
   286  
   287  // removeChild removes a context from its parent.
   288  func removeChild(parent Context, child canceler) {
   289  	p, ok := parentCancelCtx(parent)
   290  	if !ok {
   291  		return
   292  	}
   293  	p.mu.Lock()
   294  	if p.children != nil {
   295  		delete(p.children, child)
   296  	}
   297  	p.mu.Unlock()
   298  }
   299  
   300  // A canceler is a context type that can be canceled directly. The
   301  // implementations are *cancelCtx and *timerCtx.
   302  type canceler interface {
   303  	cancel(removeFromParent bool, err error)
   304  	Done() <-chan struct{}
   305  }
   306  
   307  // closedchan is a reusable closed channel.
   308  var closedchan = make(chan struct{})
   309  
   310  func init() {
   311  	close(closedchan)
   312  }
   313  
   314  // A cancelCtx can be canceled. When canceled, it also cancels any children
   315  // that implement canceler.
   316  type cancelCtx struct {
   317  	Context
   318  
   319  	mu       sync.Mutex            // protects following fields
   320  	done     chan struct{}         // created lazily, closed by first cancel call
   321  	children map[canceler]struct{} // set to nil by the first cancel call
   322  	err      error                 // set to non-nil by the first cancel call
   323  }
   324  
   325  func (c *cancelCtx) Done() <-chan struct{} {
   326  	c.mu.Lock()
   327  	if c.done == nil {
   328  		c.done = make(chan struct{})
   329  	}
   330  	d := c.done
   331  	c.mu.Unlock()
   332  	return d
   333  }
   334  
   335  func (c *cancelCtx) Err() error {
   336  	c.mu.Lock()
   337  	err := c.err
   338  	c.mu.Unlock()
   339  	return err
   340  }
   341  
   342  func (c *cancelCtx) String() string {
   343  	return fmt.Sprintf("%v.WithCancel", c.Context)
   344  }
   345  
   346  // cancel closes c.done, cancels each of c's children, and, if
   347  // removeFromParent is true, removes c from its parent's children.
   348  func (c *cancelCtx) cancel(removeFromParent bool, err error) {
   349  	if err == nil {
   350  		panic("context: internal error: missing cancel error")
   351  	}
   352  	c.mu.Lock()
   353  	if c.err != nil {
   354  		c.mu.Unlock()
   355  		return // already canceled
   356  	}
   357  	c.err = err
   358  	if c.done == nil {
   359  		c.done = closedchan
   360  	} else {
   361  		close(c.done)
   362  	}
   363  	for child := range c.children {
   364  		// NOTE: acquiring the child's lock while holding parent's lock.
   365  		child.cancel(false, err)
   366  	}
   367  	c.children = nil
   368  	c.mu.Unlock()
   369  
   370  	if removeFromParent {
   371  		removeChild(c.Context, c)
   372  	}
   373  }
   374  
   375  // WithDeadline returns a copy of the parent context with the deadline adjusted
   376  // to be no later than d. If the parent's deadline is already earlier than d,
   377  // WithDeadline(parent, d) is semantically equivalent to parent. The returned
   378  // context's Done channel is closed when the deadline expires, when the returned
   379  // cancel function is called, or when the parent context's Done channel is
   380  // closed, whichever happens first.
   381  //
   382  // Canceling this context releases resources associated with it, so code should
   383  // call cancel as soon as the operations running in this Context complete.
   384  func WithDeadline(parent Context, d time.Time) (Context, CancelFunc) {
   385  	if cur, ok := parent.Deadline(); ok && cur.Before(d) {
   386  		// The current deadline is already sooner than the new one.
   387  		return WithCancel(parent)
   388  	}
   389  	c := &timerCtx{
   390  		cancelCtx: newCancelCtx(parent),
   391  		deadline:  d,
   392  	}
   393  	propagateCancel(parent, c)
   394  	dur := time.Until(d)
   395  	if dur <= 0 {
   396  		c.cancel(true, DeadlineExceeded) // deadline has already passed
   397  		return c, func() { c.cancel(true, Canceled) }
   398  	}
   399  	c.mu.Lock()
   400  	defer c.mu.Unlock()
   401  	if c.err == nil {
   402  		c.timer = time.AfterFunc(dur, func() {
   403  			c.cancel(true, DeadlineExceeded)
   404  		})
   405  	}
   406  	return c, func() { c.cancel(true, Canceled) }
   407  }
   408  
   409  // A timerCtx carries a timer and a deadline. It embeds a cancelCtx to
   410  // implement Done and Err. It implements cancel by stopping its timer then
   411  // delegating to cancelCtx.cancel.
   412  type timerCtx struct {
   413  	cancelCtx
   414  	timer *time.Timer // Under cancelCtx.mu.
   415  
   416  	deadline time.Time
   417  }
   418  
   419  func (c *timerCtx) Deadline() (deadline time.Time, ok bool) {
   420  	return c.deadline, true
   421  }
   422  
   423  func (c *timerCtx) String() string {
   424  	return fmt.Sprintf("%v.WithDeadline(%s [%s])", c.cancelCtx.Context, c.deadline, time.Until(c.deadline))
   425  }
   426  
   427  func (c *timerCtx) cancel(removeFromParent bool, err error) {
   428  	c.cancelCtx.cancel(false, err)
   429  	if removeFromParent {
   430  		// Remove this timerCtx from its parent cancelCtx's children.
   431  		removeChild(c.cancelCtx.Context, c)
   432  	}
   433  	c.mu.Lock()
   434  	if c.timer != nil {
   435  		c.timer.Stop()
   436  		c.timer = nil
   437  	}
   438  	c.mu.Unlock()
   439  }
   440  
   441  // WithTimeout returns WithDeadline(parent, time.Now().Add(timeout)).
   442  //
   443  // Canceling this context releases resources associated with it, so code should
   444  // call cancel as soon as the operations running in this Context complete:
   445  //
   446  // 	func slowOperationWithTimeout(ctx context.Context) (Result, error) {
   447  // 		ctx, cancel := context.WithTimeout(ctx, 100*time.Millisecond)
   448  // 		defer cancel()  // releases resources if slowOperation completes before timeout elapses
   449  // 		return slowOperation(ctx)
   450  // 	}
   451  func WithTimeout(parent Context, timeout time.Duration) (Context, CancelFunc) {
   452  	return WithDeadline(parent, time.Now().Add(timeout))
   453  }
   454  
   455  // WithValue returns a copy of parent in which the value associated with key is
   456  // val.
   457  //
   458  // Use context Values only for request-scoped data that transits processes and
   459  // APIs, not for passing optional parameters to functions.
   460  //
   461  // The provided key must be comparable and should not be of type
   462  // string or any other built-in type to avoid collisions between
   463  // packages using context. Users of WithValue should define their own
   464  // types for keys. To avoid allocating when assigning to an
   465  // interface{}, context keys often have concrete type
   466  // struct{}. Alternatively, exported context key variables' static
   467  // type should be a pointer or interface.
   468  func WithValue(parent Context, key, val interface{}) Context {
   469  	if key == nil {
   470  		panic("nil key")
   471  	}
   472  	if !reflect.TypeOf(key).Comparable() {
   473  		panic("key is not comparable")
   474  	}
   475  	return &valueCtx{parent, key, val}
   476  }
   477  
   478  // A valueCtx carries a key-value pair. It implements Value for that key and
   479  // delegates all other calls to the embedded Context.
   480  type valueCtx struct {
   481  	Context
   482  	key, val interface{}
   483  }
   484  
   485  func (c *valueCtx) String() string {
   486  	return fmt.Sprintf("%v.WithValue(%#v, %#v)", c.Context, c.key, c.val)
   487  }
   488  
   489  func (c *valueCtx) Value(key interface{}) interface{} {
   490  	if c.key == key {
   491  		return c.val
   492  	}
   493  	return c.Context.Value(key)
   494  }
   495  

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